Photosensitive elements which can be used in image-reproduction processes are well-known in the graphics arts industry. Such elements are exposed to actinic radiation through an image-bearing transparency, such as a color separation transparency, and developed to produce an image which is either positive or negative with respect to the transparency used. Positive-working elements produce an image which is a duplicate of the transparency through which they are exposed. Negative-working elements produce an image that is the reverse of the transparency through which they are exposed. After imagewise exposure, the photosensitive elements may be developed by washout of soluble image areas, toning tacky image areas with a colorant, peeling apart photoadherent layers, or combinations of these techniques. A series of images may be combined to form a color proof. A useful reference for color proofing methods is Principles of Color Proofing, by Michael H. Bruno, GAMA Communications, Salem, NH, 1986.
In Cohen and Fan, U.S. Pat. No. 4,282,308, there is described a photohardenable element which is capable of producing reverse, colored images by a dry process without the need for toning with a colorant. The element comprises, in order from top to bottom, a strippable coversheet, a photoadherent layer containing a colorant, a tacky essentially nonphotosensitive elastomeric layer, and a support. After imagewise exposure to actinic radiation through the coversheet, the element can be peeled apart by peeling off the coversheet with the exposed areas of the colored photoadherent layer adherent thereto. The unexposed areas of the colored photoadherent layer remain on the supported elastomeric layer. When the imagewise exposure is through a negative transparency, and the exposed element is peeled apart, a positive image is obtained on the peeled-off coversheet and a negative image is obtained on the supported elastomeric layer. Imagewise exposure through a positive transparency, followed by peeling apart of the element, yields a negative image on the peeled-off coversheet and a positive image on the supported elastomeric layer.
Taylor, U.S. Pat. No. 4,489,154, describes a dry process for preparing a surprint proof without the need for toning with a colorant. This process uses an imagewise exposed, peel-apart photosensitive element comprising a strippable coversheet; a photoadherent layer containing a colorant; an essentially nonphotosensitive elastomeric layer; and a support. After exposure to actinic radiation through a positive image, e.g., a positive separation transparency, and peeling off the coversheet, a positive colored, photoadherent image remains on the supported elastomeric layer. This image is then adhered to a substrate. By repeating this process with different colored photoadherent layers, a multicolored surprint proof can be built up on the substrate. If exposure is through a negative image, e.g., a negative separation transparency, a positive image is obtained on the coversheet.
It is sometimes desired to have a multilayer, peel apart photosensitive element in which the adhesive relationship of the layers is the reverse of that described above. After exposure, the unexposed areas of the photosensitive layer would adhere more strongly to the coversheet than to the elastomeric layer so that the unexposed, unhardened areas would be removed with the coversheet and the exposed, hardened areas would remain on the elastomeric layer. If the photosensitive layer were colored, exposure through a positive separation transparency would produce a positive, colored image directly on the coversheet. A series of such coversheets bearing different colored images could be combined to form a full color positive overlay proof. Such precolored elements would also be particularly useful for the preparation of negative-working, multicolor surprint proofs without the use of toners, and without the additional complication of an image transfer step, or the addition of an adhesive layer.
While it may be possible to change the adhesive relationships of the unexposed and exposed photohardenable layer, this is not an easy task. The multilayer, peel-apart photosensitive element requires a very delicate balance of adhesive/cohesive forces and other properties in order to function properly. The addition of new chemicals or the substitution of new chemicals for ones previously used may change the adhesive/cohesive properties but may, at the same time, adversely affect other important properties of the system. It is, therefore, desirable to achieve reversal of the adhesion properties, i.e., photorelease as opposed to photoadhesion, by a simple alteration of the coversheet.